1. LIGHT & DARK REACTIONS
OF PHOTOSYNTHESIS

2. How photosynthesis works
“Dark
Reactions”
Light Dependent
Reactions
Light Independent
Reactions
Sugars
H2O O2
Photosystem I Photosystem II
Non-cyclic é flow
Cyclic é flow
CO2
Water Splitting
Reactions
Carbon Fixation

3. Step 1: Photoexcitation
The structure of chlorophyll is very important to its function:
Notice the alternating double bonds
These é are said to be ‘delocalized’
The polar chlorophyll head is found mixed with the phospholipids of the thylakoid membrane
This is the where photosynthesis starts...

4. Step 1: Photoexcitation
Before a photon of light strikes chlorophyll, its é are at their lowest energy level
‘ground state’
When a photon hits, an é gains energy
Becomes ‘excited’
When an excited é returns to its original state it can:
Emit light  ‘fluorescence’
Transfer é to another é carrier  ‘primary é acceptor’

5. Cool Note
If you separate a chlorophyll molecule from the thylakoid membrane...
The excited é will fluoresce as its energy lowers back to its ground energy
The red fluorescence in the middle of the jellyfish comes from chlorophyll in the ingested algae

6. Step 2: Photosystems Photosystems consist of: Antenna complex
Reaction centre
composed of a # of chlorophyll molecules and accessory pigments
Embedded in the thylakoid membrane
Photon is absorbed and transfers energy between pigments until it reaches chlorophyll a

7. Step 2: Photosystems Photosystems consist of: Reaction Centre
Antenna complex
Reaction centre
Reaction Centre
An é on chlorophyll a absorbs energy from the antenna complex and becomes ‘excited’
A redox reaction transfers the excited é to the primary é acceptor
*There are 2 Photosystems: Photosystem I Photosystem II

8. Step 3: Non-cyclic é flow
Process:
A photon strikes the antennae complex of photosystem II to excite an é
The excited é is captured by the primary é acceptor pheophytin
Through a series of redox reactions, é is transferred to plastoquinone (PQ) and then to the electron transport chain
The é powers a H+ pump allowing 4H+ to enter

9. Step 3: Non-cyclic é flow
At the same time, A ‘Z’ protein splits water into oxygen, H+ ions (protons), and é
The é from water are used to replenish the é lost in photosystem II

10. Step 3: Non-cyclic é flow
Photosystem I  NADPH
Two photons excite 2 é from photosystem I
é from photosystem I pass through another ETC containing ferredoxin, Fd
Finally, Fd, gives its é to NADP reductase that uses H+ from the stroma to reduce NADP+ to NADPH
NADPH is used in the Calvin cycle

11. Step 3: Non-cyclic é flow
*** Meanwhile, there is a H+ gradient forming in the lumen that allows ATP to be produced as H+ ions pass though ATP synthase
We have produced NADPH and ATP from non-cyclic é flow!

12. Step 3: Non-cyclic é flow

13. Step 3: Non-cyclic é flow
VIDEO

14. cyclic é flow
In some cases, excited é can take a cyclic pathway that stays within photosystem I
The excited é is picked up by Fd  b6-f  Pc
In the end, the excited é is returned to the chlorophyll it came from
This process adds to the H+ gradient to help produce ATP

15. cyclic é flow
In some cases, excited é can take a cyclic pathway that stays within photosystem I
The excited é is picked up by Fd  b6-f  Pc
In the end, the excited é is returned to the chlorophyll it came from
This process adds to the H+ gradient to help produce ATP

16. How photosynthesis works
“Dark
Reactions”
Light Dependent
Reactions
Light Independent
Reactions
Sugars
H2O O2
Photosystem I Photosystem II
Non-cyclic é flow
Cyclic é flow
CO2
Water Splitting
Reactions
Carbon Fixation

17. Calvin Cycle (Dark Reactions)

18. Step 4: Carbon Fixation
3 CO2 molecules that are absorbed through the stomata and spongy mesophyll cells are added to 3 molecules of RuBP
Ribulose – 1,6 – Bisphosphate (5-carbon molecule)
Together they form unstable 6-carbon compound
Which, instantly breaks down into 6 PGA molecules
Often called C3 photosynthesis as the first product contains 3-carbon
C4 plants exist as well

19. Step 5: Reduction Reactions
Each of the 6 PGA molecules is phosphorylated by ATP
Producing 1,3-BPG
Then, NADPH gives each 1,3-BPG 2 é
REDUCING THEM to G-3-P
ONE of the six G-3-P molecules exits the cycle to produce sugars

20. Step 6: RUBP REGENERATION
We have five 3-carbon G3P’s left
We need to regenerate RuBP so we can continue the Calvin Cycle
The five G3P molecules rearrange to REFROM the three 5-carbon RuBP
Phosphorylation by 3 ATP molecules will finally regenerate the RuBP

21. Calvin Cycle TOtals ATP USED = 9 NADPH USED = 12
ATP/NADPH are produced in the light reactions
In the light reactions,
3 ATP and 2 NADPH are produced
Conveniently, the Calvin cycle uses 3 ATP and 2 NADPH per CO2

22. Calvin Cycle TOtals ATP USED = 9 NADPH USED = 12
ATP/NADPH are produced in the light reactions
In the light reactions,
3 ATP and 2 NADPH are produced
Conveniently, the Calvin cycle uses 3 ATP and 2 NADPH per CO2

23. How photosynthesis works
“Dark
Reactions”
Light Dependent
Reactions
Light Independent
Reactions
Sugars
H2O O2
Photosystem I Photosystem II
Non-cyclic é flow
Cyclic é flow
CO2
Water Splitting
Reactions
Carbon Fixation